1. Field of the Invention
The invention relates to the bending of glass sheets, and in particular the bending on a hollowed out shape or configuration referred to as a bending frame or skeleton.
2. Discussion of the Background
The bending of glass sheets on a skeleton is widely used for bending glass sheets and in particular for simultaneously bending two glass sheets to be used in forming a bent laminated glazing such as a car windshield. Such a frame-bending method permits the production of glazing having complex curvatures and more particularly a double curvature. In this method, the two superimposed glass sheets, with an adequate interposed separating agent, are supported along their marginal end portions in a substantially horizontal manner by a frame having the desired profile, i.e., the profile corresponding to the final profile of the two bent glass sheets. Supported in this way, the glass sheets pass into a bending furnace, generally a tunnel furnace. The bending furnace is generally a tunnel furnace having heating zones at different temperatures.
The first of these zones is generally a preheating zone in which the glass sheets are heated to a glass temperature close to the softening point. The following zone is the bending zone where the glass sheets, heated to a temperature of approximately 600.degree.C., will progressively bend by gravity in order to adapt at the periphery to the shape of the frame and achieve the desired curvatures. The glass sheets are then cooled so that, on leaving the tunnel furnace, they are removed from the shaping frame. This skeleton bending process is entirely satisfactory, particularly when the curvatures are not very pronounced and/or when the bending is essentially cylindrical. When the curvatures are more pronounced, particularly in the vicinity of the periphery of the glazings, it can occur that there is a so-called counterbending at the corners of the glass sheets and in general at two of these corners or even at the four corners as a function of the geometry of the glazing, i.e., its final or definitive curvatures and also its initial cutting shape.
Counter-bending corresponds to an undesired inversion in this curvature case. The counterbent glass sheets obtained no longer meet the requirements imposed on the glazing to permit installation in a car body bay. To obviate such a counterbending, EP-A-448 447 proposes a gravity bending process having at least two stages.
The first bending stage consists of the gravity bending of the glass sheets in accordance with a first shape corresponding to a blank of the final shape. During this first stage, the glass sheets are supported by a first peripheral line provided by the bending frame.
The second bending stage consists of the gravity bending of the glass sheets in accordance with the final shape while being supported by a second peripheral line from the same frame. For this purpose, the frame is constituted on at least one portion or section of its periphery by a duplicated structure, i.e., a structure comprising selectively usable parts.
This bending process is satisfactory for the bending of two glass sheets to be used in the production of a glazing such as the windshield of an existing car. However, it has a limit linked with the so-called "docking" angle, i.e., the angle formed by a tangential vector to the side end of the widest part of the glazing with the horizontal plane, when the bent glazing is placed in a horizontal position.
The bending process described hereinbefore makes it possible to produce glazings having docking angles with a maximum value of approximately 45.degree. for small radii of curvature. However, present demands more particularly with respect to car windshields are directed at docking angles exceeding 50.degree. and which can even reach 80.degree..
In order to obviate the counter-bending, in the case of the bending of only one glass sheet, which is to be thermally tempered or toughened and for which purpose it is heated to a high level, U.S. Pat. No. 3,511,628 proposes holding the edges of said sheet during the bending operation. With such a process it would appear possible to bend a glass sheet having docking angles exceeding 50.degree.. However, this process is not satisfactory in the case of bending several glass sheets simultaneously.
EP-B-250 311 proposes the use of a force, in addition to the force of gravity, applied to the upper face of the glass sheet above the location subject to counter-bending. Such a process would also appear to be able to facilitate the production of docking angles in excess of 50.degree.. However, this process is not satisfactory for all glazing shapes which are subject to undesired counter-bending during their passage through bending furnaces.